1. Technical Field
The present invention relates to a fuel cell, particularly, to a direct methanol fuel cell (DMFC) using liquid methanol as fuel.
2. Related Art
A fuel cell converts chemical energy of fuel directly into electrical energy and outputs it by electrochemically oxidizing fuel such as hydrogen and methanol in the cell. Unlike the thermal power generation, the fuel cell does not produce NOx, SOx, or the like by combustion of fuel, therefore the fuel cell has been noticed as a clean electric energy source.
The fuel cell needs a structure for balancing supply of fuel and movement of produced reactant in order to improve the cell output. For example, JP-A 2001-283875 (KOKAI) discloses a technique of providing a layer containing water-repellent materials such as fluorine resin, silicon resin and polyethylene in a fuel electrode or an oxygen electrode.
Further, JP-A 2005-174607 (KOKAI) discloses a polymer electrolyte fuel cell having a structure for balancing permeability to gas and moisturizing property in a gas diffusion electrode. Particularly, a direct methanol fuel cell (DMFC) can be more reduced in size and weight than the other fuel cells such as a proton exchange membrane fuel cell (PEMFC) for gas fuel using hydrogen as fuel. Recently, various studies on the direct methanol fuel cell as a power source for a laptop, a cellular phone, or the like have been carried out.
A basic reaction in DMFC is as follows.Anode: CH3OH+H2O→CO2+6H++6e−  (Formula 1)Cathode: (3/2)O2+6H++6e−→3H2O  (Formula 2)
As shown in Formula 1, methanol and water molecule are required in the anode. Six protons and six electrons are produced, with carbon dioxide as waste, from one molecule of methanol and one molecule of water by use of, for example, an alloy catalyst containing mainly platinum and ruthenium. The electrons are used as output power by passing through an external electrical circuit.
As shown in Formula 2, oxygen, protons, and electrons are required in the cathode. The six electrons react with the six protons having passed through a proton-conductive electrolyte membrane and 3/2 molecules of oxygen in the cathode, and then produce 3 molecules of water as waste.
In the anode of the known DMFC, used is a coated carbon paper, which is produced by coating slurry in which hydrophobic fluoropolymer and carbon powder are mixed See, JP-A 2005-514747 (KOKAI).